Preparation Of Polystyrene Supported Chiral Salen Mn(III) Complexes And Their Catalytic Performance On The Asymmetric Epoxidation Of Olefins

A chiral Salen-Mn(Ⅲ) complex was attached on the surface of polystyrene resin by a multi-step grafting method to form a heterogenized catalyst. The structures of the intermediates and the supported complex were characterized by 1H NMR, FT-IR, elementary analysis and AAS(atomic absorption spectrometer). Then the supported complex catalyst was used in the asymmetric epoxidation of dihydronaphthalene, cis-β-methylstyrene,α-methylstyrene and styrene using m-CPBA and NaClO as oxidants respectively under the typical Jacobsen conditions. The effects of mesh size of the resin, molar ratio of catalyst to substrate, reaction temperature and time on the performance of the immobilized catalyst were investigated. Furthermore the recyclability of the polymer-supported catalyst was studied and the possible reason for the decline of catalytic activity of the catalyst was discussed. Experimental results showed that both the catalytic activity and the enantiomeric excess of the heterogenized catalyst were obviously decreased and only modest activity of the heterogenized catalyst was obtained. It was found that reusability of the immobilized catalyst is very limited. The catalyst can only be run for 2 times in the m-CPBA system and for 3 times in the NaClO system. The possible reason is the loss of Mn derived from the decomposition of the Salen complex under the reaction conditions.Based on the above results, other 3 heterogenized catalysts were prepared by changing the Salen ligand structure as the above preparation method. These supported catalysts were also used in the enantioselective epoxidation of unfunctionalized olefins under identical reaction conditions. From the experimental results, a supported catalyst with higher activity was obtained. When using Cis-β-methylstyrene as substrate and NaClO as an oxidant running at 0℃for 3 hours, the e.e. of the epoxide could reach up to 77.3%. However, the recyclability of the catalyst is still not good as expected. It was also found that the combination effect of steric and electronic properties of the substituent at C1″, C2″, C5 and C5'positions of the Salen ligand influences the performances of catalyst. In addition, the substituents at C3 and C3'positions have more effect on the catalytic activity of the catalyst.A unchiral Salen-Mn(Ⅲ) complex was immobilized on the polystyrene resin by the forementioned way. The polymer-supported catalyst and its precursors were characterized by FT-IR, elementary analysis, AAS and 1H NMR. Its performances on the aerobic epoxidation of unfunctional olefins were studied. Under the optimized conditions the conversion of cyclohexene and the selectivity of epoxycyclohexane could reach up to 91.4% and 86.3% respectively. Relative good results were also obtained when using styrene as the substrate. However, the performance of immobilized catalyst on the epoxidation of long-chain aliphatic alkenes, such as 1-octene, is usually poor. The recycle test showed that it can be reused for 3 times with some lose of its catalytic activity. It can be concluded that the decomposition of the supported complex in the catalytic run is the main reason for the activity decrease of the catalyst.